This invention relates High Density Polyethylene (HDPE) films having improved barrier properties. More particularly, the invention relates to HDPE films containing hydrocarbon resins having improved moisture barrier, and the process of making said films.
Polyolefins are plastic materials useful for making a wide variety of valued products due to their combination of stiffness, ductility, barrier properties, temperature resistance, optical properties, availability, and low cost.
The use of terpene and hydrogenated hydrocarbon resins as modifiers for polypropylene (PP) converted into oriented film is well known. Some of the attributes assigned to the use of low molecular weight resin products in polypropylene films, include good optical properties, improved processing when making oriented films, better sealing characteristics, and desirable mechanical properties and converting characteristics.
The use of hydrocarbon resins (HCR) for improving the moisture barrier properties of oriented polypropylene is also well known. The effectiveness of resin for improving barrier properties is expected to be highly dependent on the characteristics of the PP itself. These characteristics include the degree of crystallinity of the PP, the compatibility of the resin with the polypropylene amorphous regions and the amorphous region""s glass transition.
Additionally, it has been generally known that high levels of hydrocarbon resin were required to cause substantial improvements in barrier properties of polypropylene film, typically in the range 5% to 25% by weight. However, adding resin at these levels typically embrittles non-oriented PP film to an excessive degree. In oriented polypropylene (OPP) film, the orientation imparted to the polymer offsets the negative effect of the resin on ductility, so that films with good mechanical properties can be produced at the high loadings of hydrocarbon resin required to impart improvements in barrier properties.
Because of differences between ethylene polymers and polypropylene in crystallinity level, glass transition temperature, and amorphous character (linear vs. branched aliphatic structure), the effects of hydrocarbon resins in polyethylene films can not be strictly predicted based on analogy with oriented polypropylene films. Additionally, because most polyethylene films possess a relatively low degree of molecular orientation as compared to OPP films, the ability to incorporate hydrocarbon resins in polyethylene films at an effective level without ruining mechanical properties is an area of concern.
High density polyethylene (HDPE) is nominally a linear homopolymer of ethylene containing few branch points in the polymer chain. As a result of its regular structure, HDPE is a highly crystalline material with a peak crystalline melting point typically around 135xc2x0 C. Various types of HDPE are characterized by the density of the material, which ranges typically from 0.940 to 0.965 (g/cc). Density is a measure of the crystallinity developed by the HDPE material, where higher density relates to higher the level of crystallinity developed by the polymer. Mechanical properties and barrier properties are strongly influenced by the degree of crystallinity developed in the HDPE polymer.
Typical uses are in the production of blow molded containers such as milk bottles, molded articles, lightweight consumer bags and trash bags, and various types of film products.
One example of a HDPE film product is the inside liner used to package cereal products. In this and similar packaging applications, superior barrier properties of the HDPE, relative to non-oriented PP or low density PE films, is a very positive attribute of the HDPE film. One type of barrier property refers to preventing the permeation of moisture either in or out of the packaged food product.
The need exists for a method for a method for the incorporation of various hydrocarbon resins into high density polyethylene polymers (HDPE). The need also exists for films which possess superior barrier properties and still retain desirable mechanical properties such that the films can be used for packaging film applications where improved barrier properties of these films have value. Additionally, the need exists for a highly efficient process for producing films of HDPE modified with hydrocarbon resin. It has been found that by adding various types of hydrocarbon resins to HDPE polymer to form a blend, and forming a film from the blend, a superior packaging film can be produced with improved moisture barrier properties than films produced from the HDPE polymer by itself. These improved barrier films have value in packaging applications where a reduced rate of moisture loss (or gain) increases the shelf life of the packaged material. In the opposite sense, by improving the barrier properties of the HDPE film the thickness of the film used to package a material may be reduced, lowering the amount of packaging material required, and, as a result, reducing the amount of refuse derived from packaging film.
A polyethylene film comprising about 3% to about 25% by weight of a resin and about 97% to about 75% by weight of a polyethylene. The resin has a weight average molecular weight Mw of less than about 10,000 Daltons, as determined using size exclusion chromatography (SEC) using polystyrene as a standard. Resins of Mw less than 5,000 Daltons being preferred, for example resins having Mw of at least about 500 Daltons to about 2,000 Daltons being most preferred. The polyethylene has a density in the range from about 0.940 to about 0.970 g/cc, as measured at 23xc2x0 C. according to ASTM D1505. Barrier properties improve as density or crystallinity of the, preferably from about 0.940 to about 0.965 g/cc. Preferably, the film comprises about 3% to about 15% of the hydrocarbon resin.
The resin further comprises hydrocarbon resin derived by thermally polymerizing olefin feeds rich in dicyclopentadiene (DCPD). Alternatively, the resin may be hydrocarbon resin derived from the polymerization of a C9 hydrocarbon feed stream. Either of the above hydrocarbon resins may be either fully or partially hydrogenated.
Alternatively, the hydrocarbon resin may be resin derived from polymerization of pure monomers, wherein the pure monomers are selected from the group consisting of such as styrene, a-methylstyrene, 4-methylstyrene and vinyltoluene.
Alternatively, the resin may be produced from terpene olefins.
The polyethylene film may comprises a cast film or an oriented film. If the polyethylene film comprises an oriented film, it may comprise a monoaxial or biaxial oriented film. The biaxially oriented film may be produced through a blown film process or through a tenter frame orientation process.
The invention also relates to a masterbatch for the production of polyethylene films, its preparation and the use of the masterbatch wherein the masterbatch comprises a resin and an ethylene polymer wherein the resin has a weight average molecular weight Mw of less than about 10,000 Daltons, as determined using size exclusion chromatography (SEC) using polystyrene as a standard. Resins of Mw less than 5,000 Daltons being preferred, for example resins having Mw of at least about 500 Daltons to about 2,000 Daltons being most preferred. The ethylene polymer has a density in the range from about 0.87 to about 0.965, as measured at 23xc2x0 C. according to ASTM D1505. The masterbatch further comprises about 2% to about 25% by weight ethylene polymer and about 98% to about 75% by weight resin. Preferably, the masterbatch comprises about 70 to about 80% by weight resin.
The invention also relates to a process of producing a polyethylene film comprising the steps of a) blending a polyethylene with a resin to form a blend, and b) extruding the blend to form a film. The film comprises about 3% to about 25% by weight of a resin and about 97% to about 75% by weight of a polyethylene wherein the resin has a weight average molecular weight Mw of less than about 10,000 Daltons, as determined using size exclusion chromatography (SEC) using polystyrene as a standard, preferably resins of Mw less than 5,000 Daltons, more preferably resins of Mw at least about 500 Daltons to about 2,000 Daltons. The polyethylene has a density in the range from about 0.95 to about 0.965 g/cc, as measured at 23xc2x0 C. according to ASTM D1505.
Preferably, in the process of producing the polyethylene film, the resin is a hydrocarbon resin and is added to the film in the form of a masterbatch wherein the masterbatch comprises a hydrocarbon resin and an ethylene polymer wherein the hydrocarbon resin has a Mw of less than about 5000 Daltons and wherein the ethylene polymer has a density in the range from about 0.87 to about 0.965 g/cc. Preferably, the masterbatch further comprises about 2% to about 25% by weight ethylene polymer and about 98% to about 75% by weight hydrocarbon resin.
Most packaging films made from HDPE polymers are produced by a blown film or cast film process. Less is produced by the orientation process used to make OPP packaging films from polypropylene. This invention also relates to methods for improving the barrier properties of HDPE films by incorporation of resins into the polymer formulation. Additionally this invention relates to a method for incorporating resins into the HDPE film. Because of low melt viscosity of applicable resins (relative to the HDPE polymer) blending resin into HDPE during film forming process is difficult and special addition techniques are necessary.
It is known that various types of resins, including hydrocarbon resins, may be added to PP polymer formulations which are subsequently converted into oriented film (OPP film) to improve barrier properties of the modified film. The preferred resins for this application are fully hydrogenated products derived from the polymerization of various olefin hydrocarbon feedstocks. Examples of useful resin products are Regalites(copyright) R-125 resin (Hercules Incorporated, Middelburg, The Netherlands), made by hydrogenating a polymerization product derived from a C9 hydrocarbon feedstock, Piccolytes(copyright) C 125 resin (Hercules Incorporated, Wilmington, Del.) produced by polymerizing a terpene feedstock comprised primarily of limonene, or hydrogenated resins derived from the thermal polymerization of a dicyclopentadiene (DCPD) rich feedstock, such as Plastolyn(copyright) 140 resin (Hercules Incorporated, Wilmington, Del.) or Escorez(copyright) 5320 resin (Exxon Chemical Corporation). In these instances, resin has a particular interaction with the amorphous part of the PP polymer into which the resin combines, which reduces the ability of moisture to permeate through the polymer.
Also, it is well known to use hydrocarbon resins in OPP to improve barrier properties has been specifically addressed, where the orientation imparted during film production also dramatically affects barrier properties. As an example, an oriented PP film typically exhibits barrier properties 2.0 to 2.5 times better than the barrier properties of the same PP polymer converted into a non-oriented film. Additionally by orienting a PP film, its strength and ductility in the stretch directions are dramatically improved so that the brittleness effect caused by adding substantial amounts of low molecular weight hydrocarbon resin may be overcome.
The use of hydrocarbon resins to improve the barrier properties of polypropylene converted into oriented film is known. However the ability to achieve improved barrier by this method in non-oriented film constructions is poorly defined because incorporating substantial amounts of resin into non-oriented films typically reduce ductility to a level that makes the film product impractical for use. The ability to improve the barrier properties of packaging films made from HDPE or polymers other than polypropylene by using hydrocarbon resins is also poorly defined. The present invention describes how to produce useful HDPE packaging films with superior barrier properties by incorporating hydrocarbon resins, and teaches an effective, cost efficient method for producing these improved films.
HDPE packaging films with improved barrier properties can be produced by the process of melt blending hydrocarbon resin into HDPE polymer to form a blend and extruding blend into a film. HDPE polymers useful in this invention have a density in the range from about 0.940 to 0.970, where products falling in the range about 0.955 to about 0.965 are preferred. Similarly, HDPE of utility in the instant invention may have a melt index (190xc2x0 C., 2.3 kg. load, as determined by ASTM D-1238) in the range of about 0.1 to about 100 dg./min., but polymers having a melt index between about 0.5 to about 10.0 dg./min. are most preferred for the extrusion processes used to produce the packaging films of this invention. The HDPE films can be made by the cast film process or by the blown film process commonly used to fabricate HDPE packaging films. Other film fabricating techniques suitable for making HDPE packaging films can also be used to produce the films of this invention (e.g., tenter frames).
Hydrocarbon resins (HCR) of utility in this invention are low molecular weight materials derived by polymerizing an olefin feedstock. These resins have a weight average molecular weight Mw of less than about 10,000 Daltons, as determined using size exclusion chromatography (SEC) using polystyrene as a standard. Resins of Mw less than 5,000 Daltons being preferred, for example resins having Mw of at least about 500 Daltons to about 2,000 Daltons being most preferred. The resins may be derived from crude olefin feeds derived from petroleum cracking such as C5 olefin streams, C9 olefin streams, or olefin streams rich in DCPD. The resins may also be produced from terpene olefins, such as limonene derived from citrus products. The resins may also be derived from pure monomer streams such as styrene or methyl styrene monomers. Aliphatic type resins are preferred. Hydrogenated resins with little residual aromatic character are also preferred.
Among the benefits of the instant invention is the production of packaging films with improved moisture barrier properties. Moisture barrier can be measured by the ASTM E-96 method where the moisture vapor transmission rate (MVTR) of films are tested at 100xc2x0 F., 90% relative humidity. By modifying HDPE films with hydrocarbon resins, reductions in MVTR from 10% to 50% over non-modified films can be achieved.
Because resins are typically friable, dusty materials with low Mw and low melt viscosity, it is difficult to add them to HDPE during the extrusion process in the production of films. An effective method to incorporate resin into HDPE is to first form a masterbatch having a high concentration of resin combined with a polymer carrier. This masterbatch can then be added to the HDPE polymer. The resin in the masterbatch is subsequently blended in with the HDPE polymer during film extrusion. A preferred masterbatch formulation should have as high a resin loading as possible, have good handling characteristics, process well, and be blended well when added to the HDPE polymer during the film forming step.
The invention relates to improved films for packaging applications produced from HDPE polymers where the films exhibit improved barrier properties than conventional films produced from HDPE alone. The improvement consists of incorporating an effective amount of hydrocarbon resin into the HDPE polymer in order to reduce the moisture permeability through the film by more than about 10%, more typically to reduce moisture transmission by about 20% to about 40%. These films are particularly useful for packaging food products which can be negatively affected by either excessive loss of moisture under dry conditions or moisture gain under humid conditions. The invention also relates to a process for producing these films by use of resin masterbatch formulations which can be used to add hydrocarbon resin to a polymer directly during a film forming step.
HDPE polymers used to produce the films of the invention may have a density in the range from about 0.940 to about 0.970, where HDPE polymers having a density falling in the range of about 0.955 to about 0.965 are preferred. The density or the HDPE polymers are as measured at 23xc2x0 C. according to ASTM D1505. Barrier properties improve as density or crystallinity of the HDPE polymer increases, and for this reason materials with the highest density practical are preferred. The HDPE polymer may have a melt index (190xc2x0 C., 2.3 kg. load) in the range of about 0.1 to about 100 dg./min., but polymers having a melt index between about 0.5 to about 10.0 dg./min. are most preferred.
Any process suitable for producing films from HDPE polymers may be used in this invention. The films of this invention can be made by an extrusion casting process where polymer is extruded through a slit die onto a casting roll and the polymer is drawn to the final film thickness while in the molten state. The films can also be produced by the blown film process where the polymer is extruded into a cylindrical tube construction which is expanded to the final film thickness using internal air pressure inside the molten polymer tube to expand its dimensions. These are the most common fabricating methods for making HDPE packaging film, although other modified film fabricating techniques such as tenter orientation process can be used to produce the films of this invention.
The resin products useful in this invention can be any low molecular weight polymer derived by polymerizing an olefin feedstock, where the weight average molecular weight (Mw)of the material is less than about 20,000 Dalton. Suitable resins have a Mw of less than about 10,000, with hydrocarbon resins of Mw less than 5,000 being preferred, for example resins of at least about 500 to about 2,000 Mw. Mw of the resins are determined using size exclusion chromatography (SEC) using polystyrene as a standard. The resins can be derived from crude olefin feeds produced in the petroleum cracking process. Examples of these crude olefin feeds include a light olefin fraction having an average carbon number of 5 carbon atom per olefin molecule (C5 feeds) or cyclic olefins having an average of 9 carbon atom per olefin molecule (C9 feeds). Resins produced from olefin streams rich in DCPD derived from ethylene cracking can also be used effectively in this invention. Useful resins can also be produced from terpene olefins, such as limonene derived from citrus products. Lastly the resins derived from polymerization of pure monomer streams consisting of styrene, a-methylstyrene, 4-methylstyrene and vinyltoluene can be utilized in this invention. In order to be compatible in HDPE the resin should be essentially aliphatic in character, and for this reason hydrogenated resins with little residual aromatic character are desired. Fully hydrogenated resin products are preferred because of their light color and thermal stability.
One example of a resin useful in this invention is the resin derived from the polymerization of a crude C9 feed stream, followed by catalytic hydrogenation. C9 feedstock is defined as the olefin stream produced during petroleum cracking comprised of hydrocarbon olefin components having about 9 carbon atoms per molecule. Examples of olefins found in a C9 feed include but are not limited to styrene, xcex1-methylstyrene, indene, various methyl substituted indenes, 4-methylstyrene, xcex2-methylstyrene, ethylstyrene, among other olefins. The resultant resin product is aromatic in character, but can be converted to an aliphatic type resin by catalytic hydrogenation. By hydrogenation is meant that residual olefin groups in the resin and the aromatic units in the resin, are converted to saturated species by reduction with hydrogen. Hydrogenation reactions be carried under various conditions, examples being at temperatures in the range of about 150xc2x0 C. to about 320xc2x0 C., using hydrogen pressures between about 50 to about 2000 psi. More typically, the hydrogenation would be carried out at temperatures between about 200xc2x0 C. to about 300xc2x0 C. to produce the desired product. A typical catalyst for hydrogenating these resins would be Ni metal supported on a carrier such as carbon black. In this class, the preferred type of product would be a resin having more than about 90% of the aromatic units hydrogenated, preferably greater than about 95% of the aromatic units hydrogenated. Examples of this type of resin are Regalite(copyright) R-125 resin available from Hercules Incorporated, Middelburg, The Netherlands or Arkons(copyright) P-125 resin available from Arakawa Chemical Co.
Another example of a resin effective in this application are resins derived from polymerization of pure monomers such as styrene, xcex1-methylstyrene, 4-methylstyrene, vinyltoluene, or any combination of these or similar pure monomer feedstocks. The product produced by this polymerization is aromatic in character, but can be converted to an aliphatic type resin by catalytic hydrogenation. The process used to hydrogenate these resins is similar to the process, described above, suitable for hydrogenating resins derived from C9 olefin feedstocks. These resins, derived from hydrogenating oligomers of pure monomers, can be hydrogenated to various degrees, where between about 20% to about 100% of the aromatic groups in the resin are reduced to saturated units. Preferably greater than about 90% of the hydrogenated units should be hydrogenated, and more preferred is a degree of hydrogenation greater than about 95%. Examples of these resins are Regalrez(copyright) 1139 resin or Regalrez(copyright) 1126 resin available from Hercules Incorporated.
Resins useful in this invention can be derived from the polymerization of terpene olefins, examples being the cationic polymerization of monomers such as xcex1-pinene, xcex2-pinene, or d-limonene. These resins are aliphatic-type materials and hydrogenation is not required to achieve aliphatic character. However, hydrogenation to saturate residual olefin groups in the resin can be carried out to produce resins with greater thermal stability which can be likewise used as part of this invention. Examples of resins of this type include Piccolytes(copyright) A-135 and Piccolyte(copyright) C-125 resins available from Hercules Incorporated.
The most preferred resin for this invention are resins derived by thermally polymerizing olefin feeds rich in dicyclopentadiene (DCPD). Resins of this type can be produced by thermally reacting olefin streams containing between about 50% to about 100% DCPD at temperatures in the range of about 200xc2x0 C. to about 325xc2x0 C. to produce resin products which can be hydrogenated to form fully saturated materials with weight average molecular weight (Mw) values below about 5000 Daltons. Hydrogenation of these resins is not required, but is greatly preferred to achieve a low color DCPD resin product with good thermal stability. As an example, a DCPD feed containing nominally 85% DCPD, can be converted into a resin product by heating the DCPD to temperatures in the range about 260xc2x0 C. to about 300xc2x0 C., for a suitable time, typically in the range of about 10 to about 200 minutes, depending on temperature, to produce a resin which after hydrogenation and stripping to remove volatile components exhibits a Ring and Ball (RandB) softening point in the range of about 100xc2x0 C. to about 170xc2x0 C., as determined by ASTM D28-67, aliphatic character, and Mw less than about 5000. An example of this type of resin is Plastolyns(copyright) 140 resin available from Hercules Incorporated or Escorez(copyright) 5340 resin available from Exxon Chemical Corporation.
Films of the instant invention, which comprise a blend of HDPE and a hydrocarbon resin, exhibit favorable barrier properties. One way to produce films of the instant invention is to add hydrocarbon resin flakes or prills directly to the HDPE molding pellets, and convert the blend directly into a film using the film casting extruder as a mixing device to melt and blend the two components. However because of the friable, dusty nature of hydrocarbon resin products and the low viscosity of these materials at normal plastics processing temperatures, this technique is difficult to practice in commercial applications. Dust problems, and extrusion problems associated with processing blends containing more than 5% hydrocarbon resin, using single screw extruders that are typically used to make HDPE films, make the process troublesome.
Another process for making blends of hydrocarbon resin with HDPE is to combine the ingredients at the proportion desired in the final film, and melt compound the blends using an extruder or batch mixing device capable of blending the ingredients despite the large viscosity mismatch between the resin and the HDPE. Typically this compounding is done in a facility capable of dealing with dusts. After blending the materials, the molten blend is extruded through a multi-hole die and converted into solid pellet form by cooling and cutting the extrudate using typical techniques such as strand pelletization or underwater pelletization. These compounded blends are suitable for being extruded and converted into films under typical commercial conditions. A drawback of this technique is that all the material converted into film has to be run through the compounding step with its associated costs. This process does not provide the film producer with flexibility to alter the amount of resin in the final film.
Another effective way to incorporate hydrocarbon resin into HDPE films is by producing a masterbatch comprised of a high concentration of the resin in a polymeric carrier. By compounding resin in with a polymer dusting of the resin is minimized. Additionally, the polymer blended with the resin increases the melt viscosity so that the masterbatch has a much higher melt viscosity at film processing temperatures than the resin alone. By modifying the viscosity in this manner, the masterbatch can be processed more like a conventional polymer, and it can be compounded more readily into the HDPE polymer during the film extrusion process than the hydrocarbon resin alone can. An important aspect of this invention is the development of novel masterbatch formulations which can manufactured in an economically effective manner for this type application.
Hydrocarbon resin masterbatches of the instant invention comprise from about 60% to about 80% resin, and which may be made with high compounding efficiencies. Preferrably, masterbatches made with high compounding efficiencies which contain from 70% to 80% hydrocarbon resin. These improved masterbatches can be made by using ethylene polymers in combination with the hydrocarbon resin in the masterbatch.
In masterbatches of the instant invention, hydrocarbon resins may be compounded with polyolefins at a level from about 60% to about 80%, preferrably from about 70% to about 80%. Under these conditions, the relative rheological properties of both components are a critical condition for achieving good compounding efficiency. As an example, when polypropylene is the carrier, mixing of the ingredients cannot be achieved until the blend is at about 165xc2x0 C., representing the melting point of polypropylene at which the polymer turns sufficiently plastic that it can be processed. However at this temperature, the hydrocarbon resin has a very low melt viscosity relative to the polypropylene. The physical act of blending the materials is difficult because of rheological mismatch, requiring excessive mixing time and intensity to blend the materials.
Because low Mw hydrocarbon resins exhibit a very severe viscosity/temperature dependence, small changes in the temperature at which blending occurs can greatly increase the viscosity of the resin and increase the efficiency with which the resin can be blended in with the polymer. Most resins useful for modifying polyolefins have a softening point falling in the range from about 100xc2x0 C. to about 140xc2x0 C. In order to achieve high compounding efficiency when making masterbatches containing greater than 60% hydrocarbon resin, the crystalline polyolefin polymer used in the masterbatch must have a crystalline melting point no greater than about 10xc2x0 C. higher than the Ring and Ball (RandB) softening point of the resin, (obtained through use of ASTM D28-67). Within this constraint, it is desired to use ethylene polymers as a carrier, where the ethylene polymer has a crystalline melting point typically in the range of about 120xc2x0 C. to about 140xc2x0 C., and a crystallinity level, as exemplified by the density of the polymer or polymer blend used in the masterbatch, falling in the range 0.87 to 0.965 g/cc. Polymers derived from 1-butene are also suitable because of their low melting point, as are other 1-olefin polymers or copolymers with a melting point falling in the range from about 100xc2x0 C. to about 140xc2x0 C.
The ability of a resin/polymer blend to rapidly crystallize to a solid form is also a critical criteria for high compounding efficiency. Polymers such as polypropylene and polybutene crystallize very slowly when compounded into hydrocarbon resins at levels greater than about 60%, and as a result it is difficult to form pellets from these blends at high rates. In contrast, polyethylene type crystallinity develops very rapidly in these blends, even when the polymer concentration is as low as about 20% in the final blend. This fast solidification promotes high compounding efficiencies. As a result, it is preferred that some amount of polyethylene type crystallinity be present in these improved resin masterbatches.
The desired level of crystallinity developed by the polyolefin in these resin masterbatches can be between 10% to 70% based on polymer, and depends on the on both the final application for the masterbatch and the process used to pelletize the masterbatch. When underwater pelletization is used, and fast and complete solidification of the compound to a hard pellet is acceptable and desired, a high crystallinity polymer can be used. In strand pelletization, where the strand must develop resistance to stretching very rapidly, but the strand must remain fairly ductile to prevent breaks, a polymer or polymer blend with an intermediate crystallinity level is desired. In some applications, such as to achieve optimal barrier properties, it is desirable to maximize the crystallinity in the final blend, and as a result high crystalinity in the polymer used in the masterbatch is desired.
The polymer used in these masterbatch formulations can have a melt index (MI) (190xc2x0 C., 2.16 kg. load) between 0.1 to 10 dg./min. Materials having a higher MI are easier to mix in with the low Mw resin. However blends using a lower MI polymer (higher Mw) have higher melt strength and higher melt viscosity, and as a result are easier to form into strands or pellets, and also have better processing characteristics when the masterbatch is blended in with a polymer during film processing. Because of these conflicting constraints, the polymer used in the masterbatch preferrably has a MI between 0.5-5.0 dg./min.
The modified HDPE films of this invention can be produced by incorporating the resin into the HDPE polymer using any of the techniques described above. The preferred method is the masterbatch method, where compounds containing 50% to 80% resin, preferrably 60% to 80% resin, are added to HDPE polymer to form a blend from which the films are directly produced. It is desired that the polymer used in the masterbatch does not detract from the barrier properties of the film, and as a result the preferred polymer is a crystalline polyethylene polymer having a density greater than about 0.91 g./cc.
The HDPE films of this invention exhibit moisture barrier properties which are 10% to 50% better than comparable films containing no resin made from the same HDPE polymer under the same film forming conditions. Because some resins are more effective than others, the required resin addition level depends on the barrier improvement desired and the resin type used.
Examples of hydrocarbon resins used with good effect in HDPE films include MBG 273(trademark) hydrogenated C9 resin, available from Hercules Incorporated, Middelburg, The Netherlands, Regalrez 1139 hydrogenated styrene-vinyl toluene copolymer resin, and Plastolyn(copyright) 140 hydrogenated DCPD resin produced by the thermal polymerization of DCPD monomer, both available from Hercules Incorporated, Wilmington, Del. The preferred resin is the hydrogenated product derived from the resin product formed by thermally polymerizing DCPD feedstocks. Examples of this preferred resin type include Plastolyns(copyright) 140 resin available from Hercules Incorporated and Escorez(copyright) 5300 and Escorez(copyright) 5320 resins available from Exxon Chemical Co.
The hydrocarbon resins cam be incorporated into the HDPE films at levels from 3% to 25%, but the preferred level of modification is by incorporating from 3% to 15% hydrocarbon resin into the film. Increasing the resin content typically causes further improvements in the barrier properties of the film, however at the sacrifice of some of the mechanical properties of the HDPE film. The optimal resin add level is typically a trade-off between these two effects.
The following examples will serve to illustrate the invention, parts and percentages being by weight unless otherwise indicated.